1. Field of the Invention
This invention relates to lipophilic cationic compounds and several of their uses. The invention also relates to a novel DNA transfection method, in which the compounds of this invention can be used.
2. Related Art
Liposomes are microscopic vesicles consisting of concentric lipid bilayers. Structurally, liposomes range in size and shape from long tubes to spheres, with dimensions from a few hundred Angstroms to fractions of a millimeter. Regardless of the overall shape, the bilayers are generally organized as closed concentric lamellae, with an aqueous layer separating each lamella from its neighbor. Vesicle size normally falls in a range of between about 20 and about 30,000 nm in diameter. The liquid film between lamellae is usually between about 3 and 10 nm.
Typically, liposomes can be divided into three categories based on their overall size and the nature of the lamellar structure. The three classifications, as developed by the New York Academy Sciences Meeting, "Liposomes and Their Use in Biology and Medicine," of December 1977, are multi-lamellar vesicles (MLV's), small uni-lamellar vesicles (SUV's) and large uni-lamellar vesicles (LUV's).
SUV's range in diameter from approximately 20 to 50 nm and consist of a single lipid bilayer surrounding an aqueous compartment. Unilamellar vesicles can also be prepared in sizes from about 50 nm to 600 nm in diameter. While unilamellar are single compartmental vesicles of fairly uniform size, MLV's vary greatly in size up to 10,000 nm, or thereabouts, are multi-compartmental in their structure and contain more than one bilayer. LUV liposomes are so named because of their large diameter which ranges from about 600 nm to 30,000 nm; they can contain more than one bilayer.
Liposomes may be prepared by a number of methods not all of which produce the three different types of liposomes. For example, ultrasonic dispersion by means of immersing a metal probe directly into a suspension of MLV's is a common way for preparing SUV's.
Preparing liposomes of the MLV class usually involves dissolving the lipids in an appropriate organic solvent and then removing the solvent under a gas or air stream. This leaves behind a thin film of dry lipid on the surface of the container. An aqueous solution is then introduced into the container with shaking in order to free lipid material from the sides of the container. This process disperses the lipid, causing it to form into lipid aggregates or liposomes.
Liposomes of the LUV variety may be made by slow hydration of a thin layer of lipid with distilled water or an aqueous solution of some sort.
Alternatively, liposomes may be prepared by lyophilization. This process comprises drying a solution of lipids to a film under a stream of nitrogen. This film is then dissolved in a volatile solvent, frozen, and placed on a lyophilization apparatus to remove the solvent. To prepare a pharmaceutical formulation containing a drug, a solution of the drug is added to the lyophilized lipids, whereupon liposomes are formed.
A variety of methods for preparing various liposome forms have been described in the periodical and patent literature. For specific reviews and information on liposome formulations, reference is made to reviews by Pagano and Weinstein (Ann. Rev. Biophysic. Bioeng., 7, 435-68 (1978)) and Szoka and Papahadjopoulos (Ann. Rev. Biophysic. Bioeng., 9, 467-508 (1980)) and additionally to a number of patents, for example, U.S. Pat. Nos. 4,229,360; 4,224,179; 4,241,046; 4,078,052; and 4,235,871.
Thus, in the broadest terms, liposomes are prepared from one or more lipids. Though it has been thought that any type of lipid could be used in liposomes, e.g. cationic, neutral or anionic lipids, experience with positively charged liposomes has indicated several problems which have not been fully addressed to date. The amines which have to date been employed in preparing cationic liposomes have either not been sufficiently chemically stable to allow for the storage of the vesicle itself (short shelf life) or the structure of the amines has been such that they can be leached out of the liposome bilayer. One such amine, stearylamine, has toxicity concerns which limit its use as a component of liposomes in a pharmaceutical formulation. Another amine, dimethyl dioctadecyl ammonium bromide, lacks the appropriate molecular geometry for optimum formation of the bilayers that comprise the liposome structure.
Various biological substances have been encapsulated into liposomes by contacting a lipid with the matter to be encapsulated and then forming the liposomes as described above. A drawback of this methodology, commonly acknowledged by those familiar with the art, is that the fraction of material encapsulated into the liposome structure is generally less than 50%, usually less than 20%, often necessitating an extra step to remove unencapsulated material. An additional problem, related to the above, is that after removal of unencapsulated material, the encapsulated material can leak out of the liposome. This second issue represents a substantial stability problem to which much attention has been addressed in the art.
Liposomes have been used to introduce DNA into cells. More specifically, various DNA transfection methodologies have been used, including microinjection, protoplast fusion, liposome fusion, calcium phosphate precipitation, electroporation and retroviruses. All of these methods suffer from some significant drawbacks: they tend to be too inefficient, too toxic, too complicated or too tedious to be conveniently and effectively adapted to biological and/or therapeutic protocols on a large scale. For instance, the calcium phosphate precipitation method can successfully transfect only about 1 in 10.sup.7 to 1 in 10.sup.4 cells; this frequency is too low to be applied to current biological and/or therapeutic protocols. Microinjection is efficient but not practical for large numbers of cells or for large numbers of patients. Protoplast fusion is more efficient than the calcium phosphate method but the propylene glycol that is required is toxic to the cells. Electroporation is more efficient then calcium phosphate but requires a special apparatus. Retroviruses are sufficiently efficient but the introduction of viruses into the patient leads to concerns about infection and cancer. Liposomes have been used before but the published protocols have not been shown to be any more efficient than calcium phosphate. The most desirable transfection method would involve one that gives very high efficiency without the introduction of any toxic or infectious substances and be simple to perform without a sophisticated apparatus. The method that we describe satisfies all of these criteria.